What kind of Mac is it, which version of OS 9 (OS 9, OS 9.1, OS 9.2?) and is it using SCSI or IDE? If SCSI, is the ID number one that's already being used?

Harddrive doesn't boot or recognize with the Apple utility on the OS install CD, but I did put it in a Mac that takes multiple drives and it mounted.

This is a little confusing. If the drive is mounted, then the OS does recognize it. The Apple Drive Utility (can't remember the name for the life of me) is supposed to be for specific brands. If you want something a little more powerful and useful, see if you can get FWB HardDisk Toolkit for OS 9.

Sadly enough, it's been a while from my Mac support days. Anyways, a couple of ideas. First, if you can get it to mount with the other Mac - do a full backup NOW if you haven't done it already!

It could be a disk surface problem, a wiring problem (SCSI chain termination for example can be fuzzy sometimes), or a problem with configuring the drive (duplicate SCSI address; other cabling systems can have slave/master switches etc.) If it turns out to be the surface, you may have to low-level format and restore from backup and even that may be a short-term help or no help at all.

If this is supposed to be a boot disk in a system that has any significance at all (and it turns out to be a disk surface problem), I would probably replace the drive just to be sure.

Yeah, the first iMac models were really crappy.
If I remember correctly, such a simple thing as the serial number was hidden away behind a plastic lid at the bottom of the unit!
I used to work for Apple tech support, and we had to get the serial number from every machine people called about. It could take many minutes just to find the serial number...